It's my understanding that it's not technically illegal for people to visit Cuba, it's just illegal to spend money there. The regulations say travel-releated TRANSACTIONS, not travel in general. That's how they get you -- try traveling someplace without spending any money! But, if Michael Moore did it through a third-party and can prove it, they really can't get him for anything.
Also, you should know that the United States government is playing a dangerous game here. It has not been settled that the government can tell it's citizens where to go (or not go). Almost every case where people are willing to take this further in the courts, rather than pay the fine, the US drops the charges. Why? Because there is a chance it is found unconstitutional, and then the government would not be able to enforce it's so-called 'travel ban'.
I can't understand why some of you 'patriots' who say you support America revel in glee in 'getting' Michael Moore over something as petty as this, when he only exercising his basic right to go where he wants to go, and associate with whoever he wants to. So long as he's not doing it in a manner that harms US interests, what's the big deal? I think you should be more concerned about China, which we have basically handed over our economy over too, and has much more influence over us than some lil-old Carribbean island that's been stuck in the 1950s.